The present invention relates to apparatus for destroying contaminants such as volatile organic compounds in an air or other gas stream prior to release to the atmosphere.
More specifically, the invention relates to the incineration or oxidation of such contaminants in a reversing flow-type incinerator and to the system for preheating a bed of material having heat retention and heat exchanging properties.
Many manufacturing operations produce waste gases or exhaust air which include environmentally objectionable contaminants, generally combustible fumes such as solvents and other hydrocarbon substances, e.g., gasoline vapors, paint fumes and chlorinated hydrocarbons. The most common method of eliminating such combustible fumes prior to emitting the exhaust gases to the atmosphere is to incinerate the waste gas or exhaust air stream.
One method of incinerating the contaminants is to pass the waste or exhaust air stream through a fume incinerator prior to venting the waste gas or exhaust air stream into the atmosphere. An example of a fume incinerator for incinerating combustible fumes in an oxygen bearing process exhaust stream is disclosed in U.S. Pat. No. 4,444,735. In such a fume incinerator, the process gas stream is passed through a flame front established by burning a fossil fuel, typically natural gas or fuel oil, in a burner assembly disposed within the incinerator. In order to improve efficiency, it may be desirable to preheat the process exhaust stream prior to passing it through the flame front.
Another type of incinerator commonly used for incinerating contaminants in process exhaust streams is the multiple-bed, fossil fuel fired regenerative incinerator, such as, for example, the multiple-bed regenerative incinerators disclosed in U.S. Pat. No. 3,870,474. In the typical multiple-bed systems of this type, two or more regenerative beds of heat-accumulating and heat-transferring packing material are disposed about a central combustion chamber equipped with a fossil fuel fired burner. The process exhaust stream to be incinerated is passed through a first bed, then into the central combustion chamber for incineration in the flame produced by firing auxiliary fuel therein, and then discharged through a second bed. As the incinerated process exhaust stream passes through the second bed, it loses heat to the material making up the bed. After a predetermined interval, the direction of gas flow through the system is reversed such that the incoming process exhaust stream enters the system through the second bed, wherein the incoming process exhaust stream is preheated prior to entering the central combustion chamber, and discharges through the first bed. By periodically reversing the direction of gas flow, the incoming process exhaust stream is preheated by absorbing heat recovered from the previously incinerated process exhaust stream, thereby reducing fuel composition.
A somewhat more economical method of incinerating combustible contaminants, such as solvents and other hydrocarbon based substances, employs a single regenerative bed as disclosed in U.S. Pat. No. 4,741,690. In the process presented therein, the contaminated process exhaust stream is passed through a single heated bed of heat absorbent material having heat-accumulating and heat-exchanging properties, such as sand or stone, to raise the temperature of the contaminated process exhaust stream to the temperature at which combustion of the contaminants occurs, typically to a peak preheat temperature of about 900.degree. C., so as to initiate oxidation of the contaminants to produce carbon-dioxide and water. Periodically, the direction of flow of the process exhaust stream through the bed is reversed. As the contaminants cornbust within the center of the bed, the temperature of the process exhaust stream raises. As the heated exhaust stream leaves the bed, it loses heat to the heat-accumulating material making up the bed and is cooled to a temperature about 20.degree. C. to 50.degree. C. above the temperature at which it entered the other side of the bed. By reversing the direction of the flow through the bed, the incoming contaminated process exhaust stream is preheated as it passes that portion of the bed which has just previously in time been traversed by the post-combustion, hot process exhaust stream, thereby raising the temperature of the incoming process exhaust stream to the point of combustion by the time the incoming process exhaust stream reaches the central portion of the bed.
In the prior art single regenerative bed incinerator, means are provided in the middle portion of the bed to heat the bed to the desired self-combustion or self-decontamination temperature during start-up and prior to the passage of any of the contaminated gases. This is usually done by means of an electric heater embedded in the middle of the bed of packing material. The problem associated with this system is that the use of electricity to preheat the packing material is expensive and requires a considerable period of time to heat up the mass of heat transfer material.